Wednesday, October 11, 2006

Some Lost+Found gyan from JMM :)

“While the JMM specified in chapter 17 of the Java Language Specification was an ambitious attempt to define a consistent, cross-platform memory model, it had some subtle but significant failings. The semantics of synchronized and volatile were quite confusing….

...JSR 133, chartered to fix the JMM...

Most programmers know that the synchronized keyword enforces a mutex (mutual exclusion) …. But synchronization also has another aspect: It enforces certain memory visibility rules as specified by the JMM. It ensures that caches are flushed when exiting a synchronized block and invalidated when entering one, (significant performance degradation on multiprocessors) so that a …. It also ensures that the compiler does not move instructions from inside a synchronized block to outside…. The JMM does not make this guarantee in the absence of synchronization -- which is why synchronization (or its younger sibling, volatile) must be used whenever multiple threads are accessing the same variables.”

I’m not sure if this cache flush is entire cache flush (128k to 2m) or cache-line flushes, but i consider it really bad. All operations are stalled until memory request is complete.

I never knew this behavior of “synchronized”. I always wondered about the bad performance of concurrency primitives, some facts like these makes my understanding clearer, why it's not scalable,

This will give one certain idea of how painful it is to achieve "Platform Independence", apart from being the buzzword.